1. Field of the Invention
The present invention relates to a pipe for supplying fuel to an engine in a gasoline direct-injection engine system.
2. Description of the Related Art
In particular, the present invention relates to a pipe having resistance to inferior fuel containing a corrosive component.
In recent years, in a gasoline direct-injection engine system whose development and release to a market have been advanced in an automobile industry for the purpose of reducing environmental load according to fuel consumption improvement, as shown in FIG. 1, a pipe 1 for supplying fuel (gasoline) to a direct-injection rail 30 of an engine (not shown) from a fuel tank 10 via a pump 20 has been demanded to have pressure resistance and air tightness higher than those of an existing multi point injection (MPI) engine (see Japanese Patent Application Laid-Open No. 2006-152852, Japanese Patent Application Laid-Open No. 2002-54534, and the like).
Further, similarly, according to advance of globalization of the automobile industry due to the rise of developing nations represented by People's Republic of China or India in addition to market expansion of alternate fuels such as represented by alcohol fuel derived from biological materials for the purpose of environmental load reduction, based upon operations of automobiles equipped with a gasoline direct-injection engine using fuel derived from biological materials or fuel (hereinafter, referred to as “corrosive fuel”) containing much corrosive component containing such as water, salts, and corrosive factors (mainly, acid), which were supplied under an inferior environment, a pipe having resistance even to these inferior corrosive fuels has been demanded.
In the conveying pipe of fuel (shown by reference sign 1 in FIG. 1) used in the gasoline direct-injection engine system, products obtained by performing various plastic workings (pipe-end forming working, bending working or the like) or joining working (brazing working or the like) to a stainless material (without performing a unique surface treatment owing to corrosion-resistant performance specific to the stainless material) have been most adopted as a standard, as a specification which allows stable material supply with globalization of the automobile industries, especially, manufacturing bases and has various performances such as the above-described pressure resistance, air tightness, or corrosion resistance.
Also, steel materials which are more inexpensive than the stainless materials adopted in the existing MPI engine system normally have been adopted for specific products satisfying required performances.
However, the stainless pipe which has been subjected to plastic working or joining working includes the following problems:
Stress corrosion cracking (SCC) to salt damage specific to automobile-use environment in a cold area or a marine coast region;
Corrosion resistance to a fuel having extremely high corrosiveness due to its high concentration content of water or salts (especially, corrosive cationic ions such as chloride) and use under high-pressure and high-temperature conditions in a direct-injection system or the like; and
Sensitization (rising the degree of risk of occurrence of SCC) or mechanical property lowering (lowering of strength) due to thermal influence during joining work.
Therefore, product-working conditions, materials, product specifications are limited by these problems, which results in not only increase in cost or impossibility of stable material supply due to addition of steps or requirements of high precision, but also impossibility of product realization.
Unlike such a stainless pipe, a steel pipe such as low-carbon steel has sufficiently satisfactory properties about the problems involved in the plastic working or the joining working except for the corrosion resistance. Even regarding the corrosion resistance which is only one and maximal problem, it has been known that the corrosion resistance of the steel pipe can be considerably elevated by applying anticorrosive plating having a sacrifice anti-rust mechanism of Zn series or Al series to an outer surface of the steel pipe against salt damage due to an external environment and applying anti-rust plating excellent in barrier property of Ni series, Cr series, Co series, Sn series, or the like to an inner surface of the steel pipe against corrosive fuels.
However, when only the anti-rust plating specialized in only the barrier anticorrosive mechanism is adopted against the corrosive fuels containing water or salts, it is necessary to adopt a film having a substantial film thickness (for example, a level from several tens μm to a hundred and several tens μm) or change a plurality of films to a multi-layer/complex (alloying) structure in order to completely exclude a defect such as a pinhole which may become a corrosion starting point. However, on the contrary, of course, in addition to such a point that cost increases due to the thickening of a film or the complexity, there is also such a case that breaking (cracking) or separation (delamination) of a plated film occurs due to lowering of a film strength or adhesion strength with a base material, and the barrier performance rather lowers and the corrosion resistance becomes poor so that the required performances cannot be satisfied and a product cannot be realized.
As the countermeasure to the above problem, it is thought to apply anti-rust plating having a sacrifice anticorrosion mechanism of Zn series or Al series excellent in salt damage resistance to an inner surface of the steel pipe, but occurrence of such a new problem is concerned that because the anti-rust plating has a sacrifice anticorrosion function, Zn or Al which is an anti-rust plating component is eluted in corrosive fuels as ions so that the eluted ions raise various adverse effects at various sections of an internal combustion engine.
In view of these circumstances, an object of the present invention is to provide a steel fuel conveying pipe which has a high resistance to corrosive fuel to solve such a problem, and connects a high-pressure pump and a direct-injection rail in a gasoline direct-injection engine system while maintaining reliability without impairing a function of the direct-injection engine, and a bypass pipe for coupling direct-injection rails in a V-type gasoline engine.